Cardiovascular diseases are leading causes of morbidity and mortality. One such disease is left ventricular noncompaction (LVNC), in which trabeculae fail to undergo compaction. Trabeculae are sheet-like structures extending from the myocardium to the heart lumen and function to pump efficiently. A lack of trabeculation causes embryonic lethality in mice and excess trabeculation causes cardiomyopathy and heart failure in human. Its clinical relevance is exemplified by trabecular non-compaction cardiomyopathy, in which trabeculae fail to undergo compaction and about half a million Americans suffer from this disease. Despite the fundamental nature of this morphogenic process and its critical clinical implication, the cellular and molecular mechanisms that regulate trabeculation are not clear. During trabecular initiation, myocardial cells protrude into the cardiac jelly and are oriented perpendicular to the heart wall. We deleted the intracellular adaptor protein Numb and its homolog Numblike, known as Numb Family Proteins (NFP) in mouse heart to generate myocardium NFP double knockout (MDKO) and found that MDKO hearts displayed a variety of cardiac defects, including a marked disruption in the trabecular initiation and cardiomyocyte differentiation. 3-dimentional (3D) imaging indicates that cardiomyocytes in MDKO fail to protrude into the heart lumen. Using lineage tracing/mosaic mouse model, we found that the majority of the NFP null clones failed to localize to the trabeculae. Our central hypothesis is tha NFP regulate trabecular initiation and differentiation by controlling directional migration and oriented cell division (OCD) via N-cadherin (N-CAD) dependent mechanisms, which will be tested by two aims. Aim I: Determine whether NFP regulate trabecular initiation by controlling directional migration and OCD. We will perform multicolor lineage tracing using the Brainbow mouse in which single cells in the heart tube will be genetically labeled before trabeculation, which allows us to analyze the localization of each labeled cardiomyocyte descendants and track their dynamics during trabeculation. Time-lapse 4D imaging will be applied to observe how cardiomyocytes initiate trabecula. We will identify if OCD is an asymmetric cell division and contributes to trabecular cardiomyocyte differentiation. This aim will reveal the cellular basis fo trabecular initiation and morphogenesis. Aim II: Determine whether NFP regulate trabeculation by stabilizing N-CAD. We will determine the functional residues/domains of Numb that are required for migration and N-CAD expression. Epistasis analysis and transgenic line rescuing will be applied to determine the genetic interaction between N-CAD and Numb. This aim will begin to reveal molecular mechanisms of how NFP regulate trabeculation. Completion of these aims is expected to yield novel insights into the cellular and molecular mechanisms of trabecular initiation and cardiomyocyte differentiation in vivo, and the functions of NFP in these processes, which will ultimately provide a mechanistic basis for treating non-compaction cardiomyopathy and morphogenesis related congenital heart defects (CHD).